A part of a switched capacitor circuit is shown in FIG. 1. The circuit comprises a capacitor 2 which is connected to an input pin via a transistor switch 4 and to a reference voltage, analog ground VAG, via a transistor switch 6. The capacitor is connected to an output pin, which may be connected to the input of an operational amplifier or other switching stage, depending upon the application. The gate of transistor switch 4 is coupled to receive a switch control signal PHO and the gate of transistor switch 6 is connected to receive the inverse of the switch control signal PH1. When the switch control signal is high, capacitor 2 is charged from the input pin. When the switch control signal is low, the capacitor discharges to analog ground VAG.
When the transistor switch 4 is `on`, the voltage on its gate is fixed, but the voltage on its drain and source is changing according to the signal VINP at the input pin. This means that the resistance between the source and drain of the transistor switch 4 (hereinafter the source-drain on-resistance) also varies according to the input signal. Thus, the RC constant, during charging of the capacitor 2, is changing according to the input signal VINP. This can produce non-linear distortions.
Such distortions are particularly problematical in mixed-signal integrated circuits (ICs): that is, ICs that process both analog and digital signals. The analog parts of such ICs operate in extremely noisy digital environments, due to the digital noise and voltage spikes which penetrate to the analog part and which are rapidly changing. Since the RC constant depends on the input signal, the sampled spike voltage on the capacitor also depends on the input signal. Hence the voltage on capacitor 2 depends non-linearly on the input signal, which results in non-linear distortions.
In some designs, the transistors 4 and 6 are replaced by a double transfer gate, in order to provide a larger input voltage range. However, the on-resistance of the double transfer gate also depends on the input signal, and thus, the same problems exist.
In some chips, a charge pump is used to generate a gate-to-source voltage for the transistor switch, which is substantially independent of the input signal. This ensures that the on-resistance of the transistor switch is substantially constant (neglecting the bulk effect). Since the charge pump generates voltages greater than the supply voltage, the MOS transistors, used in such a chip, suffer from electrical over-stress. This reduces the reliability of the chip.
There is therefore a need to provide an improved driver circuit which provides constant gate to source voltage in the transistor switch, whilst ensuring that the voltages are no greater than the supplied voltage.